This applies to both Screenwriter 4 and 6: Add the first extension as you would normally do by pressing '(' after the character name. Select whichever extension you want to come SECOND. Then return to the character name, press '(' again but this time choose 'add as text'. You can then type a second extension in front of the automagically generated one. Don't worry, even though the extension is indeed added as text it will be treated as an extension (i.e. not show up in character lists, etc.).

If you have Screenwriter set to automatically generate character continueds they will be added as a second extension automatically.

Hope this helps,--Yannick

P.S. Please include the Screenwriter version number you're using in future requests as some issues are handled differently in MMS4 and Scrw6.